It has been conventional for the flotation type separating means to operate such that, during flowing movement of an oil and water liquid mixture, oil particles contained therein are floated due to the difference in specific gravity between oil and water and the oil particles are aggregated or flocculated on the surfaces of parallel plates. Accordingly, it is desirable in this case for the mixture of oil and water to flow through narrow-spaced parallel plates in the form of a laminar flow and its retention time is made as long as possible. For complying with these requirements, the apparatus becomes large in size and complicated in manufacture.
If an oil particle becomes very fine, the floating ability thereof is decreased and resistance to its floating is increased and the influence of the flowing movement acting thereon is also increased, and thus its floating becomes extremely slow. Accordingly, it has been hitherto difficult to decrease the remaining oil content to a great extent by the floating type separating means alone.